1. Field of the Invention
The present invention relates to a toner for use in electrophotography. The present invention also relates to an image forming method and a process cartridge using the toner.
2. Discussion of the Background
High-speed and high-grade full-color image forming apparatuses have been developed in the electrophotographic industry recently. Unexamined Japanese Patent Application Publication No. (hereinafter JP-A-) 07-209952 and JP-A-2000-075551 each disclose so-called tandem image forming methods in which different-color toner images are formed on multiple electrophotographic photoreceptors (hereinafter simply “photoreceptors”) provided in tandem. The different-color toner images are superimposed on a single intermediate transfer member, in a process called “primary transfer process”, and the resultant composite toner image is transferred onto a recording medium, in a process called “secondary transfer process”. Tandem image forming methods generally use an intermediate transfer member. The use of an intermediate transfer member is advantageous in that even when non-image areas on photoreceptor are contaminated with fouling, the fouling may not be transferred onto a recording medium directly. However, it is disadvantageous in that total transfer efficiency of toner images is low because toner images are subjected to transfer processes twice.
On the other hand, to respond to increasing demands for higher-grade full-color images, toners are required to be much smaller, so as to reproduce latent images more faithfully. Japanese Patent No. (hereinafter JP-) 3640918 and JP-A-06-250439 disclose toner production methods employing polymerization which are capable of controlling the size, shape, and surface structure of toner. Because toners produced by polymerization methods (hereinafter “polymerization toner”) typically have a small size and a controlled shape, polymerization toners may produce high quality images with a low pile height (i.e., thickness of an image) and high reproducibility of dots and thin lines.
Small-size toner particles may exhibit large non-electrostatic adhesion forces to photoreceptors and/or intermediate transfer members. Therefore, small-size toner particles may have low transfer efficiency, especially in the secondary transfer process in high-speed full-color image forming apparatuses. This is because not only non-electrostatic adhesion forces between toner particles and an intermediate transfer member are large, but also multiple toner particles are superimposed on one another while being subjected to an electric field (hereinafter “secondary transfer electric field”) in the secondary transfer process for a very short time, disadvantageously, in high-speed apparatuses.
To increase transfer efficiency in the secondary transfer process, one proposed approach involves increasing the strength of the secondary transfer electric field. However, if the secondary transfer electric field is strengthened excessively, transfer efficiency may decrease adversely. Another proposed approach involves widening the secondary transfer nip so that toner particles may be subjected to the secondary transfer electric field for a longer time. To widen the secondary transfer nip when a bias roller applies voltage to an intermediate transfer member by contact therewith, there may be only two possible approaches which involve increasing the contact pressure of the bias roller and increasing the diameter of the bias roller. However, increasing the contact pressure of the bias roller may degrade the resultant image quality, and increasing the diameter of the bias roller may cause upsizing of apparatuses. On the other hand, to widen the secondary transfer nip when a charger applies voltage to an intermediate transfer member without contact therewith, one possible approach involves increasing the number of chargers. All the approaches described above have limitations especially in high-speed apparatuses, and therefore widening of the secondary transfer nip for the purpose of improving transfer efficiency is considered to be substantially impossible.
In attempting to decrease adhesion forces between toner particles and photoreceptors/intermediate transfer members, JP-A-2001-066820 and JP-3692829 disclose methods of adjusting the kind and amount of external additives of toners. In particular, they use large-size external additives. Such toners exhibit lower non-electrostatic adhesion forces to photoreceptors and/or intermediate transfer members. Therefore, the toners may provide high transfer efficiency, stable developing property, and high cleaning ability.
Although such toners may have high transfer efficiency in an early stage, the transfer efficiency may decrease with time because mechanical stresses are continuously applied to the toners in developing devices. In particular, external additives are buried in toner particles and do not reduce adhesion forces between the toner particles and photoreceptors and/or intermediate transfer members, resulting in decrease of transfer efficiency. In high-speed apparatuses, toners are agitated more strongly, in other words, greater mechanical stresses are applied to the toners. Therefore, burial of external additives in toner particles is accelerated.
In order to reliably keep high transfer efficiency for an extended period of time even in high-speed apparatuses, mechanical strength of the surfaces of toner particles may be required to be controllable so that external additives are not buried therein even upon application of mechanical stresses. If the mechanical strength of the surfaces of toner particles is too strong (stiff), the toner particles may be prevented from melting. In addition, in a case in which the toner includes a release agent such as a wax, the release agent may be prevented from exuding from the toner particles, resulting in deterioration of fixing ability of the toner.